set validation: validate as fine if we have null instead of iterable

Rationale: now I can skip/abort the validation conditionally such as with the
following:

  in: ->
    if @get('country') == 'CA'
      ['ON', 'QC', 'NS', 'NB', 'MB', 'BC', 'PE', 'SK', 'AB', 'NL', 'NT', 'YT', 'NU']
    else
      null
Merge pull request #350 from tremby/set-validator

set validation: validate as fine if we have null instead of iterable